The term “digital twin” has gained an ever-increasing popularity since the last five years. Digital twins promote the existence of a virtual counterpart of any physical system for monitoring the system behaviour and suggesting possible improvements automatically. Given many industries’ interest on developing and using digital twin systems, it becomes highly crucial to develop such systems with least cost and high quality. This can be addressed with the architecture-centric design approach. However, the literature still lacks in any approach that can be applied for the specifications of any digital twin system architectures and any concrete modeling notation set. In this paper, we propose an architecture modeling language called DTLang for the high-level specifications of any digital twin architectures. DTLang is based on the well-regarded C4 architecture model and thus offers 4 different architecture viewpoints which are context, container, component, and code viewpoints. Each viewpoint addresses a different concern and is related to another viewpoint for the hierarchical (i.e., traceable) specifications of digital twin system architectures. With DTLang, the viewpoint models can be specified separately using distinct graphical notation sets and also can easily be traced between each other. We demonstrate the use of DTLang via the building fire protection case-study.